This proposal describes a training program to advance my academic career in the development of strategies for the targeted radiotherapy of cancer. The purpose of this award is to encourage my independent research program, and this period will be used to expand both my scientific knowledge and mentoring skills. During the K99 award period, I will be mentored by Dr. Jason Lewis and co-mentored by Dr. David Scheinberg. Dr. Lewis is an internationally recognized radiochemist and inorganic chemist with tremendous experience in the synthesis, development, and clinical translation of radiopharmaceuticals for both the imaging and therapy of cancer. Dr. Scheinberg, the chair of the Program in Molecular Pharmacology and Chemistry at Memorial Sloan-Kettering Cancer Center, is an expert in the development and translation of targeted radioimmunotherapeutics. Both Drs. Lewis and Scheinberg have mentored many scientists and clinical fellows, several of whom have transitioned to successful academic careers. Memorial Sloan-Kettering Cancer Center (MSKCC) will provide institutional support, including the resources to conduct laboratory research, opportunities to foster career development and continuing education, and an open scientific environment to foster the interaction required for me to achieve my goals. The overall goal of this research proposal is for me to learn the fundamental theory and laboratory practice of targeted radioimmunotherapy in the context of developing a novel strategy for the pretargeted radioimmunotherapy of colorectal cancer. The proposal extends directly from my previous research experience, as I have spent a significant portion of my postdoctoral fellowship investigating the application of inverse electron-demand Diels-Alder 'click' chemistry both to the synthesis of radiopharmaceuticals for positron emission tomography (PET) and to the creation of antibody-based, pretargeted PET imaging strategies. This research has established that Diels-Alder click chemistry is an extremely effective tool for the rapid, facile, and modular construction of radiopharmaceuticals; further, this work has shown that this bioorthogonal click chemistry can be employed as the foundation of a highly effective pretargeted PET imaging strategy. The specific aim of this proposal during the 2-year K99 award period is the synthesis, chemical characterization, and in vitro biological characterization of the molecular components of the pretargeted radioimmunotherapy system, specifically the dienophile-modified, colorectal-cancer targeting huA33 antibody and the tetrazine-bearing, 90Y- and 177Lu-labeled radioligands (Specific Aim 1). The specific aims for the 3- year R00 award period extend from this aim and are to investigate the in vivo pharmacokinetics and biodistribution of the pretargeted system and its individual molecular components (Specific Aim 2) and to determine the in vivo efficacy of this pretargeted radioimmunotherapy strategy in preclinical models of colorectal cancer (Specific Aim 3).